1. Field of the Invention
The present invention relates to fiber optic Sagnac interferometers for measuring rotation rate. More particularly, this invention pertains to such a device that incorporates apparatus for light source wavelength stabilization.
2. Description of the Prior Art
U.S. Pat. No. 5,123,741 of Spahlinger issued Jun. 23, 1992 and titled "Fiber Optic Sagnac Interferometer with Digital Phase Ramp Resetting Via Correlation-Free Demodulator Control", property of the assignee herein, describes a fiber optic Sagnac interferometer for measuring rotation rate that discloses a particular modulation/demodulation scheme. In that scheme, the phase difference between the counterpropagating pair of light beams is modulated by a signal composed of a plurality of variable components and limited by a modulo operation to 2.pi.. A first signal component is a ramp generated by an integration process that compensates non-reciprocal incremental phase shifts of the two light beams. A second signal component represents a positive integral value of .pi./2 in each instance for t.sub.o, the transit time of each light beam through the coil in the neutral condition. This component comprises the sum of two components, the first component being formed from alternately successive values 0 and .pi./2, and the second component formed from values 0 and .pi. which are successive to a large extent without correlation. The detector signal is demodulated by a synchronous demodulator driven by the product of four factors. The first factor has the value -1. The second factor is -1 if the second component is 0, and is +1 if the second component is .pi.. The third factor is -1 if the second component derived from the last preceding modulation cycle was 0, and is +1 if the second component derived from the preceding modulation cycle was .pi.. The fourth factor is -1 if the first component has the value 0, but is +1 if the first component has the value .pi./2.
Fiber optic rotation rate sensors of the foregoing type with statistical modulation, controlled by a random generator for the drive signal of a phase modulator, avoid the bias defects caused by electromagnetic cross-couplings. As a consequence, insensitivity ranges, an inherent problem of all rotation rate sensors of this type and corresponding quality requirements, are avoided.
The magnitude of the measurement signal in the measurement of non-reciprocal phase shifts due to rotation with a fiber optic Sagnac interferometer for a given input rotation rate is inversely proportional to the wavelength of the light source. Accordingly, attempts have been made to stabilize the wavelength of the light source. An example of such an attempt is illustrated in FIG. 1 which discloses a light source L operated with a stabilizing electronic system SE which keeps the injection current I and/or the temperature T constant. The remaining assemblies of the fiber optic Sagnac interferometer or rotation rate sensor shown only partially in FIG. 1 are well known to those skilled in the art. These include a beam splitter ST1 irradiated by the light source L, a downstream polarizer P, a second beam splitter ST2 for splitting the polarized light beam into two identical partial light beams that are injected into the two ends of a fiber coil FS. A phase modulator PM is situated in one branch of the coil FS. The two partial light beams are recombined, after passing through the fiber coil FS, at the second beam splitter ST2. The interference pattern arising as a result of a phase shift between the two partial light beams, created by rotation of the fiber coil FS, is coupled out at the beam splitter ST1 and acts in a known manner on a photodetector PD1.
A further possibility for stabilizing the wavelength of the light source L (taught, for example, by U.S. Pat. No. 5,311,279 of Wendler issued May 10, 1994 and titled "Integrated Optical Apparatus For The Interferometric Measurement of Light Waves", property of the assignee herein, and illustrated in FIG. 2) consists of directly measuring the wavelength by means of an additional detector PD2 employing an additional unbalanced measuring interferometer M1. The interferometer M1 is trans-irradiated by a part of the light branched off at the first beam splitter ST1, and a regulating signal is derived therefrom to stabilize the wavelength of the light source L.
A fundamental disadvantage of the first method is that the wavelength is only indirectly stabilized and possible ageing effects are not covered. The second method, requiring an additional measuring interferometer, incurs a not-inconsiderable additional hardware expenditure.